1. Field of the Invention
The present invention is in the field of cable and connector components for high speed data communication. More particularly, the invention is in the field of cable and connector components in which stray reactances and cross talk are compensated for.
2. Related Art
High speed electronic communication systems wend their way throughout the landscape of our workplaces, having become a ubiquitous feature of a technological society. Telephone, computer and process control networks, local area networks (LANs), wide area networks (WANs) and the Internet, and others form the backbone of modem communication. These networks and communication systems are generally referred to hereinafter as xe2x80x9cinterconnect systems.xe2x80x9d As society seeks to send more and more data through the wires constituting these interconnect systems, data rates, the quantity of information a given wire carries in a given period of time, have been pushed higher and higher. Since the quantity of information carried as an electronic signal increases as the frequency of the electronic signal increases, all else being equal, cables for use in modern, high speed interconnect systems are designed to carry very high frequency signals, e.g. 500 MHZ, without significant cross-talk or insertion loss over useful distances, e.g. hundreds or thousands of feet.
Modern offices and factories are flexible frameworks. Spaces mutate and evolve into different uses over time, necessitating rewiring portions of the various interconnect systems used. Some cable may remain permanently installed in the walls of a building, while other cables are moved from one portion of an interconnect system to another when the system is reconfigured. Consider, for example, the cable which connects your telephone instrument to the wall jack. When you move the telephone from one room to another, you generally bring that cable with the phone. To make such flexibility possible, the cable and telephone network employ a connector system including the wall jack and a plug on the cable. For purposes of the remaining discussion, a xe2x80x9cconnector systemxe2x80x9d includes a plurality of xe2x80x9cconnectorsxe2x80x9d by which one cable or component of a system can be releasably connected for electrical communication with another. Connectors include jacks or receptacles and plugs. A connection is formed by the conductive elements of a connector system when the connectors are mated, for example when a plug is inserted into a jack.
Electrical signal connectors include, generally, a body and one or more conductive, signal carrying elements. The body may include an insulating portion which supports and protects the signal carrying elements apart from each other. The entire connector may be surrounded, optionally, by a conductive shield. The signal carrying elements include, generally, two or three portions: a contact portion, a terminal portion and optionally a connecting portion therebetween. When a connecting portion is not used, the terminal portion and the contact portion are directly connected or integral with each other.
There are a number of popular modular, multi-position connectors, including those commonly referred to as RJ-style connectors, used in both telecommunications applications and high speed data communications applications. Among the more popular configurations are 4-conductor, 6-conductor and 8-conductor types, commonly (and sometimes erroneously) referred to as RJ-22, RJ-11 and RJ-45 types, respectively. For ease of description, without loss of generality, all such multi-position connectors are referred to herein below as an 8-conductor high speed communications connector. However, more or fewer conductors, and other configurations can be used, as will be evident to those skilled in the art.
In the case of an 8-conductor high speed communications type connector, the body is generally a solid, one or two-piece plastic unit, surrounding and supporting the signal carrying elements. When constructed using lead frame technology, the signal carrying elements are stamped out of one or more sheets of thin metal. The contact portion has a blade shape in the case of an 8-conductor high speed communications plug and has a wire spring shape in the case of an 8-conductor high speed communications jack. The terminal portion may be shaped for any one of several suitable termination methods. For example, the terminal portion may be shaped as a spike, tube, wire, etc., for termination by insulation displacement (i.e., piercing), soldering or crimping, printed wiring board (PWB) attachment, etc., respectively. Conventionally, the connecting portion, if used, is simply a wire or similar conductor connecting the contact portion to the terminal portion.
Cables for transmitting high speed digital signals frequently make use of balanced differential twisted pair transmission line technology, as explained above, because balanced differential pair transmission lines avoid some types of crosstalk and other interference, i.e., noise. Balanced differential twisted pairs are referred to hereinafter as twisted pairs. Information is transmitted over a twisted pair as electrical signals. In a twisted pair, each wire of the pair carries an information signal which is equal in amplitude and 180xc2x0 out of phase with the other. That is, the signals are equal and opposite, referred to hereinafter as differential signals. Ideally, the proximity of the wires of a twisted pair to each other causes crosstalk and noise to affect both wires of the pair equally. Thus, crosstalk and noise ideally appear in both wires of the twisted pair in equal amplitudes and 0xc2x0 out of phase with each other, referred to hereinafter as a common mode signal. Receivers for use with such cables detect differential signals, while substantially rejecting common mode signals. However, there are sources of differential noise, even in a well-balanced twisted pair transmission line.
Connector elements include asymmetrical parasitic reactances which are a principle source of differential noise due to coupling. The following discussion focuses on the parasitic reactances included in RJ-style connectors, such as the 8-conductor high speed communications, 8-position connector commonly used in North America for data communication networks, although the principles of the present invention are not limited in applicability to either the 8-conductor high speed communications connector specifically or the RJ-style connectors generally. In RJ-style connectors, as shown in FIG. 1, the plug 101 includes a plurality of flat, blade-shaped terminals 103 which mate with a corresponding plurality of bent spring terminals 107 included in the jack 109. The blade-shaped terminals exhibit significant capacitive coupling, while the spring terminals exhibit significant inductive coupling.
In the body of a multi-pair cable, each wire of each pair is affected substantially equally by adjacent wires because the pair is twisted. However, when a multi-pair cable is terminated at an RJ-style plug or jack, the twisted pairs are untwisted and flattened out into a linear arrangement. Thus, some wires are adjacent wires of unrelated pairs, over a significant length. This gives rise to coupling between adjacent wires from different pairs, with an interfering signal introduced into one wire of a pair, but not into the otherxe2x80x94differential noise.
The present invention provides an improved interconnect system including a cable and connectors.
In an interconnect system according to aspects of the present invention, including a communications cable of balanced pairs of conductors, a plug and a jack, wherein one or more of the cable, the plug and the jack include a stray, reactance, the system further includes a compensating reactance built into another one or more of the cable, the plug and the jack. The compensating reactance may further comprise a xcfx80-network connected in one conductor of a first pair of conductors. In at least some embodiments, the compensating reactance corrects for both near end cross-talk and far end cross-talk. According to some embodiments using the xcfx80-network, the xcfx80-network includes components having values which when combined with the stray reactances form a symmetrically arranged network comprising: a central capacitor having a value 2C and having one side connected to a first conductor of a second pair of conductors; a pair of inductors, one of the pair of inductors being a stray inductor inherent in the interconnect system, each having a value L and connected in series with each other at a central node and each also having an end node, the central capacitor having another side connected to the central node; and a pair of end capacitors, one of the pair of end capacitors being a stray capacitor inherent in the interconnect system, each having a value C and connected from the end node of each of the pair of inductors to a second conductor of the second pair of conductors. This arrangement may also include a second symmetrical network arranged to form a balanced circuit.
A more particular embodiment of the invention includes the features described above incorporated into a lead frame structure built into the jack. The lead frame comprises: a first pair of signal conductors defining the central capacitor; a signal conductor defining one of the pair of inductors; and a second pair of signal conductors defining one of the pair of end capacitors.
Alternatively, the jack can be mounted on a printed wiring board, comprising a first inductive printed wiring trace in one layer of the printed wiring board. This embodiment can incorporate any of the above-described features in the printed wiring board. Moreover, when the cable includes balanced pairs of wires a-b and c-d, wherein a stray mutual inductance exists between wires a and c, and the mutual inductance formed by the first and second inductive printed wiring traces is between wires a and d. In this case, the second inductive trace also forms a xcfx80-network which includes components having values which when combined with the stray reactances form a symmetrically arranged network. That network comprises: a central capacitor having a value 2C and having one side connected to the second conductor of the second pair of conductors; a pair of inductors, one of the pair of inductors being a stray inductor inherent in the interconnect system, each having a value L and connected in series with each other at a central node and each also having an end node, the central capacitor having another side connected to the central node; and a pair of end capacitors, one of the pair of end capacitors being a stray capacitor inherent in the interconnect system, each having a value C and connected from the end node of each of the pair of inductors to the first conductor of the second pair of conductors.
The apparatus described above can include balanced pairs of conductors a-b and c-d and the plug can introduce a stray capacitance between conductors a and c having a value C and the jack can introduce a stray mutual inductance between conductors a and c having a value L. In this apparatus, the compensation reactance can comprise: a capacitor having a value 2C connected from conductor b to conductor c; a mutual inductor having a value L linking conductors a and c with a polarity opposite that of the stray mutual inductance; and a capacitor having a value C connected from conductor a to conductor c. This can be embodied in a lead frame structure built into the jack. The lead frame can comprise a first pair of signal conductors defining the capacitor of value 2C; a second pair of signal conductors defining the mutual inductor of value L; and a third pair of signal conductors defining the capacitor of value C.
In any of the above described embodiments, wherein the cable includes balanced pairs of wires a-b and c-d, and wherein a stray capacitance exists between wires a and c, the compensating reactance can be formed by a twist formed in a portion of the cable within the plug. The twist can introduce a compensating capacitance. Alternatively, the twist can introduce a compensating inductance. The twist may be between wire a and d. Likewise, wherein a stray capacitance exists between wires b and d, the compensating reactance can also be formed by a twist in a portion of the cable within the plug. The twist can introduce a compensating capacitance. Alternatively, the twist can introduce a compensating inductance. The twist may be between wire b and c.
In accordance with yet another aspect of the invention, embodiments may comprise a multi layer lead frame structure whose signal conductors comprise compensating capacitive and inductive structures. The capacitive structures may comprise: a first signal conductor; and a second signal conductor; the first and second signal conductors juxtaposed in space to exhibit a substantially capacitive reactance. The inductive structures may comprise a first signal conductor exhibiting a substantially inductive reactance. The inductive structures may further comprise a second signal conductor juxtaposed in space with the first signal conductor so the first and second signal conductors exhibit a mutual inductance therebetween. There may also be a second symmetrical network arranged to form a balanced circuit.
Finally, any of the above described embodiments may include a switch selectively connecting at least one reactive compensating element into the system.